invoicely will not accept duplicate invoice numbers, even if they are for two different years.
We recommend you use some kind of prefix for the starting year e.g. 2021-0001, 2021-0002, 2021-0003, etc.
Our systems are configured to automatically detect a change in your invoice numbering system. Once a new numbering style is introduced by you upon creating an invoice, invoicely will automatically keep this numbering style for future invoices you create.